Materials for nonlinear optical devices are used extensively as wavelength converting devices, optical shutters, and the like in laser transmitters and the like. Materials for nonlinear optical devices known today include crystals of inorganic substances such as KDP KH.sub.2 PO.sub.4) and lithium niobate (LiNbO.sub.3) and those of organic substances such as 2-methyl-4-nitroaniline (MNA). It is generally known that compared to inorganic crystals, organic crystals are about 10 to 100 times larger in the coefficient of second harmonic wave generation (hereinafter abbreviated as SHG), about 1000 times faster in optical response and have higher thresholds for optical damage.
SHG does not occur in single crystals having a center of symmetry. Among crystalline organic substances known today, MNA which assumes a crystal form having no center of symmetry is active for SHG and the intensity of its SHG is about 20 times as great as urea. However, MNA has a low melting point and is incapable of phase matching in crystal form. A strong need has, therefore, arisen for the development of an organic material for nonlinear optical devices which is stable and capable of readily producing a large single crystal that has no center of symmetry and which exhibits high activity for SHG.